Johann Jacoby (1 May 1805 in Königsberg – 6 March 1877 in Königsberg) was a Left-wing German-Jewish politician.
The son of Königsberg (now Kaliningrad) merchant, Gerson Jacoby, and his wife, Lea Jonas, Jacoby studied medicine at the Albertina University of Königsberg and in 1830 started practicing in his native city, but soon became involved in political activities in a liberal interest, which involved him in prosecutions and made him well known throughout Germany.
His first published brochures called for Emancipation of the Jews. Unlike some other contemporary writers on that subject, he vehemently held that granting equal right to Jews was not a special favor, but their natural right as human beings.
In later writings he called for reform of the medical services in Prussia, attacked the judicial system which he considered oppressive as well as the state censorship, and upheld "The Right of the Prussian People for a Constitution". These writings got him prosecuted on charges of Lèse majesté, but he was eventually acquitted.
His reputation as a man of fearless honesty was greatly enhanced during the revolutionary upheaval of 1848-1849. He was a delegate in both the Prussian National assembly and the All-German Frankfurt Parliament, and was reckoned among the conspicuous leaders of the Left in both of them.
A particular event connected with his name was when King Frederick William IV of Prussia refused to listen to the criticism made by deputies of the Prussian National assembly during an audience in 1848. Jacoby, one of the deputies (some accounts described him as leader of the delegation) had the courage to say to the King: It is the misfortune of Kings that they will not listen to the truth!, a saying which got widely published and soon made him very popular in the public.